MRI has achieved prominence as a diagnostic imaging modality, and increasingly as an interventional imaging modality. The primary benefits of MRI over other imaging modalities, such as X-ray, include superior soft tissue imaging and avoiding patient exposure to ionizing radiation produced by X-rays. MRI's superior soft tissue imaging capabilities have offered great clinical benefit with respect to diagnostic imaging. Similarly, interventional procedures, which have traditionally used X-ray imaging for guidance, stand to benefit greatly from MRI's soft tissue imaging capabilities. In addition, the significant patient exposure to ionizing radiation associated with traditional X-ray guided interventional procedures is eliminated with MRI guidance.
MRI uses three fields to image patient anatomy: a large static magnetic field, a time-varying magnetic gradient field, and a radiofrequency (RF) electromagnetic field. The static magnetic field and time-varying magnetic gradient field work in concert to establish both proton alignment with the static magnetic field and also spatially dependent proton spin frequencies (resonant frequencies) within the patient. The RF field, applied at the resonance frequencies, disturbs the initial alignment, such that when the protons relax back to their initial alignment, the RF emitted from the relaxation event may be detected and processed to create an image.
For imaging of soft tissue of patients with implanted medical devices, such as catheters, guidewires, stents, cardiac defibrillators (ICDs), pacemakers, neurostimulators, cochlear implants, and the like, MRI is preferable to other modalities including X-ray, computer tomography, ultrasound and positron emission tomography (PET).
Localization of medical devices during use is desirable and often required for medical procedures. For example, as a medical device is advanced through the patient's body during an interventional procedure, its progress may be tracked so that the device can be delivered properly to a target site. Once delivered to the target site, the device can be monitored to determine whether it has been placed properly and/or is functioning properly. Providing the ability to track the location of medical devices is useful in interventional procedures such as cardiac electrophysiology procedures including diagnostic procedures for diagnosing arrhythmias and ablation procedures such as atrial fibrillation ablation, ventricular tachycardia ablation, atrial flutter ablation, Wolfe Parkinson White Syndrome ablation, AV node ablation, SVT ablations and the like. Tracking the location of medical devices using MRI is also useful in oncological procedures such as breast, liver and prostate tumor ablations; and urological procedures such as uterine fibroid and enlarged prostate ablations.
Currently, several methods of locating position(s) of a medical device during a medical procedure exist. One exemplary method is a magnetic field method. In this method, a magnetic field is transmitted that permeates all non-metallic surfaces. A miniaturized sensor designed for medical applications is placed on the instrument that is inserted into the body. The location of the sensor may be determined based upon magnetic field strength and/or orientation. Another exemplary method is an impedance based method. In this method, an electric field is transmitted through the body and the bioimpedance is measured between locations. The location of a medical device or instrument may then be determined based upon the impedance variance. Another exemplary method utilizes an ultrasound transducer to provide an image of a medical device and procedural tissue used in positioning. Yet another exemplary method uses optical trackers that emit or reflect a light source that is in turn sensed by one or more detectors. The light source is typically infrared, but may alternatively operate in another frequency range as will be appreciated by those skilled in the art.
This non-exhaustive list of exemplary methods may be termed “field location” techniques. Each of these field location techniques provides spatial coordinates (i.e. x, y, z) relative to a point external to the patient. The spatial coordinates are provided in what is commonly referred to as “absolute” space. As appreciated by those skilled in the art, providing spatial coordinates in absolute space requires registration of the external point relative to the patient. Thus, one disadvantage of such field location techniques arises from the fact that if the position of the patient changes during a procedure, re-registration with respect to the external reference location is required. Another disadvantage of field location techniques is their inherent accuracy limitations due to non-ideal and/or non-homogeneous field behavior in the body.
In an attempt to overcome the disadvantages inherent in field location techniques, it is possible to utilize the MR scanner to determine the location of a tracking coil embedded in or attached to the medical device or instrument. Thus, tracking position using the MR scanner is an alternative to using field location techniques such as those previously described. MR tracking has the advantage of requiring no registration with respect to any external point or reference images generated by the MRI, as images created with MRI are referenced to so-called “patient” space. However, when MRI is utilized for both tracking and imaging, there may be a decrease in the imaging performance because tracking sequences must be time multiplexed with imaging sequences.
When used in combination with MRI, field location techniques will suffer from being referenced to absolute space rather than patient space. Patient space is a coordinate system that includes spatial warping caused by non-ideal gradient fields. For instance, assume that at some absolute point (x=y=z=0), patient and absolute space may be perfectly aligned. However, as one moves away from that point, patient space may be nonlinear or increase with a different scale as compared to absolute space. As such, circular objects imaged with MRI may appear somewhat oblong. Correction software in the MRI may be used to compensate for this effect. Such compensation is, in general, dynamic, in that different compensation is required for different images, depending upon several variables. In addition, absolute space may be offset from patent space such that registration of the two spaces is required (for example, (x=y=z=0) for absolute space may not be (x=y=z=0) for patient space, and/or the two spaces may be rotated with respect to one another.
As will be understood based on the foregoing, current technologies for tracking a medical device are inadequate. Thus, what is needed is a system and method that combines the benefits of both field location and MRI techniques to provide an improved means for locating and tracking a medical device.